Modern semiconductor integrated circuits are formed on an electrical die, which is interconnected to a substrate. Input-output (I/O) pads on one surface of the die, sometimes called die-pads, provide the electrical connection points to the circuit formed in the die. A variety of techniques used to form the electrical and mechanical bond between the die and carrier substrate are known.
The most popular techniques for interconnecting the die to the substrate are wire-bonding and flip chip attachment. In wire-bonding, the die is placed on the substrate with the die pad surface extending away from the substrate. Electrically conductive wires are used to connect the die-pads to substrate-pads (also known as bond-fingers) on the substrate. In flip chip attachments however, electrically conductive column bumps which extend from the die-pads are formed first. These column bumps are then reflowed to form solder balls. The die is then “flipped” so that the surface having the die pad faces the substrate, and the solder balls are attached to substrate-pads on the substrate.
Flip chip attachment techniques were first introduced by International Business Machines Corp. in 1960s. Flip chip attachment techniques have proven advantageous as they generally permit smaller chip packages, and better electrical and thermal performance resulting from shorter connection lengths between die-pads and substrate-pads.
In flip chip attachments, solder joints also provide both an electrical and a mechanical connection of the die to the substrate. This can sometimes be a drawback as solder joints are more susceptible to mechanical stresses which may cause solder joints to fail and electrical paths to be broken.
The die and the substrate often have different coefficients of thermal expansion. This means that when the die is bonded to the substrate using a reflow process, stress is already built into the solder joint of the bumps. When heat is invariably generated during operation of a chip package, the die and the substrate expand at different rates. This causes mechanical stress and solder-fatigue. Individual solders may disconnect from the substrate-pads, which may render the chip package inoperable.
A common technique to reduce the mechanical stress in flip chip attachments is to introduce an encapsulant or an underfill between the die and the substrate. The underfill bonds to both the die and the substrate and lowers the stress on the solder bumps during thermal expansion.
Another approach to minimize the effect of mechanical stress on flip chip attachments is to increase the size of the solder balls so that the cross-sectional area of individual connections is larger and the connections are able to withstand greater mechanical stress they can withstand. However, this requires that all solder balls are larger since the solder balls must be coplanar (i.e., must have the same height) in order to be attached to the bond-fingers reliably. Specifically, differences in solder ball heights would mean that shorter solder balls would fail to make reliable connections to bond-fingers.
This is unfortunate as the distribution of mechanical stress is not uniform across all solder balls. In particular, the mechanical stress on corner solder balls may be significantly higher than on the remaining solder balls. Increasing the size of all solder balls, however, would also decrease the pitch or I/O density for a given die size.
Accordingly, there is a need for a new method of flip chip attachment.